How do I calibrate the zero on P100?

You can find a quick video on the recalibration process:

The full instruction:

i) Undo the lock ring to access and clean the grind chamber. Any residual grind on the burr may affect the trueness of the zero you're establishing subsequently, so it's important to start with a cleaned grinder.

ii) Once the grinder is cleaned and put back together, with the motor turned off, adjust the collar finer all the way until burrs lock (i.e. collar can't move any further).

iii) Then adjust coarser by about 2 whole number on the grind scale.

iv) Set the Speed to 1 and turn on the motor (using the manual switch - not the autopurge power switch). The lowered speed can help minimise any wear and tear on the burr coating since we'll be contacting the burrs (next).

vI) Gradually adjust the collar finer until you hear burr rub noise. Then back off very slightly. Switch off the motor. This is your zero (per our definition).

vi) Rotate the number dial scale so that the number zero is aligned to the reference pointer. If required, you can also apply another sticker on the dial, so that the position of the dial is marked (that way you would know if it got moved by accident again).